Step-wise automatic transmissions are known in the practice, these transmissions are constructed, at least in a planetary transmission design or as auxiliary transmissions. Step-wise automatic transmissions, constructed in accordance with planetary design, are equipped with hydro-dynamic torque converters as starting elements as well as with frictional shifting means so that, with this type of transmission, shifting can be carried out without an interruption in continuous traction. So that the motor torque in the drive string, which is to be conducted to the output drive shaft can be transferred by the frictional shifting element, it is necessary that these frictional elements are relatively large sized. In this case, however, the shifting element proves disadvantageous, since draw-power losses when towing or large losses in the disengaged state thereof occur. Since the frictional shifting elements are generally hydraulically operated, a large and complex hydraulic circuit with a hydraulic pumping system is necessary which, because of a pick-up capacity of the pump arrangement and by leakage losses, leads to a deterioration of the efficiency of the transmission. The draw-power losses can be reduced by the insertion of a substitution for the frictional shifting element. However, in such a case, the exchange of the gear stage would then be, disadvantageously, carried out with the occurrence of traction interruptions.
In order to execute shifting, which is free of traction interruptions, with automatic step-wise transmissions, even of the auxiliary type, these, for example, must be constructed as the so-called dual-clutch transmissions. This type of transmission possesses two frictional shifting elements by which two power strings are shifted into the power flow or, alternately, may be shifted out of the power flow. In the two power strings, shape-fit, shifting element gear pairings of various ratios, have the capability of shifting “into” or “out of”. In this case, the ratios of the power strings can then be adjusted or changed, if the currently employed power string is not precisely shifted into the power flow, but rather finds itself in a load-free condition. In the area of operating, shape-fit, shifting elements, in comparison to frictional shifting elements, there appears an essentially smaller torque so that lower losses occur and efficiency in a small transmission assembly is thereby enhanced.
The dual-clutch transmissions which are built in the auxiliary manner in comparison to the planetary transmissions, however, are characterized by a lower power density; on account of this, in a case of the same number of the gear steps, a demand for more construction room space is called for. However, especially where motor vehicles are concerned, such extra space is available only in severely restricted amounts.
Further, an effective degree of gear efficiency in an auxiliary transmission lies somewhat at the lower level of hand-shifting, which is caused by a known dual-spur, gear engagement. Contrary to this, planet gear transmissions possess an efficiency, which is dependent upon the currently involved gear set scheme. This is true especially in the principally used driving gears and the transmission efficiency is higher than in the case of a coaxially designed auxiliary transmission.
DE 31 31 138 A1 discloses a, planetary gear transmission with changeable load shiftings. This is intended for motor vehicles having automatic transmission control and a plurality of coupled planetary gear sets. The vehicles are known to also have shifting clutches and shifting brakes for the formation of drive strings with different ratios in which only two shifting clutches which, for example, are installed on the motor drive shaft as load shifting clutches. These, as directed, transmit drive torque over two drive strings. The remaining shifting clutches and shifting brakes then can always be shifted, if they are not already included in the transfer of the torque. By an appropriate selection and arrangement of the coupled planetary gear sets, shifting clutches and shifting brakes in combination with the two load-shifting clutches, there arises a high multiple usage of the individual construction elements as well as a simple assembly with satisfactory ratios and a relatively high number of gear positions.
The known load-shifting, planetary transmission of DE 31 31 138 A1, however, possesses the disadvantage that its planetary gear sets are bound by stepped dual planetary gears, which are characterized by high manufacturing costs. These gears are also subject to off-axial deviations, which contribute to lessening the efficiency of the gear engagement of the planetary gear sets.
Hence, the present invention has the purpose of making a planetary transmission available with a high operational efficiency.